Portable gas detectors containing electrochemical gas sensors are well known for monitoring potentially hazardous environments, for example mines, tunnels, sewers and other closed environments. Safety regulations require that the sensors within the detector are tested on each occasion that they are taken into a potentially hazardous environment and calibrated according to manufacturer's recommendations and that is indeed good commercial practice but it is frequently not complied with for reasons of cost and time.
Currently, sensors are calibrated by passing a calibration gas of fixed, known composition from a gas bottle at a predetermined flow rate through a conduit and placing the sensor in contact with the gas flowing through the conduit. Calibration gas flowing out of the conduit is vented to atmosphere and so the procedure is wasteful of calibration gas, which is expensive. In addition, the gas flow rate in the conduit must be adjusted and controlled to pre-set levels, which is time consuming. For these reasons, calibration is often not performed as frequently as the regulations require. The gas required for calibration could be hazardous and if so, calibration should be carried out in a controlled environment.
GB-A-2049193 describes an alcohol meter that includes an arrangement for performing calibration by storing a calibration gas in a cylinder and pumping it past the sensor to the atmosphere. However, portable gas monitors that work by diffusion of the monitored gas into the monitor have no flow path past the sensor.
U.S. Pat. No. 4,489,590 describes a monitor for a flammable gas including a hood that isolates the monitor when calibration gas is applied.
U.S. Pat. No. 4,854,153 describes the calibration of a sensor by flowing calibration gas past the sensor
The present invention provides an alternative, quicker and more cost effective method of calibrating gas sensors.